A multichannel spectrophotometer is a device in which a beam of light emitted from a light source is cast into a sample to interact with this sample, the light resulting from the interaction (e.g. transmitted light or fluorescent emission) is dispersed into wavelengths by a light-dispersing element, and its intensity at each wavelength is detected. For example, such a device is used in a detection unit of a liquid chromatograph (see Patent Literature 1). FIG. 7 shows one example of the multichannel spectrophotometer used in the detection unit of a liquid chromatograph. A beam of light emitted from a light source 1 is focused by a concave mirror 2 and cast into a sample cell 3. A sample which has been temporally separated into components within a column (not shown) continuously flows into the sample cell 3 along with a mobile phase. After absorbing specific wavelengths of light, those components are discharged to a drain. The light transmitted through the sample cell 3 is reflected by a concave mirror 4. The reflected light passes through a slit 5 and is dispersed into wavelengths by a concave diffraction grating 6, to be eventually detected by a multichannel detector 7 (which is hereinafter called the “detector 7”), such as a photodiode array detector (PDA detector) or CCD linear image sensor.
The detector 7 includes a large number of micro-sized light-receiving elements arrayed in a one-dimensional form. When the light dispersed into wavelengths by the concave diffraction grating 6 falls onto the detector 7, the wavelength-dispersed light is simultaneously detected by the light-receiving elements and converted into electric signals corresponding to the intensity of light. Based on the detection signals from the detector 7, an absorption spectrum over a predetermined wavelength range can be created.
In the spectrophotometer having the previously described configuration, the light dispersed by the concave diffraction grating 6 is detected by the light-receiving elements each of which receives the light within a different wavelength range having the same wavelength width. The first and nth light-receiving elements located at both ends of the detector 7 are designed to respectively receive light at the uppermost and lowermost wavelengths λ1 and λn of the measurable wavelength range of the spectrophotometer. With this design, the light intensity at each wavelength can be accurately detected, allowing for the identification of a sample component as well as the measurement of its concentration from the peak wavelength and peak intensity of the obtained absorption spectrum.
There should normally be a linearity between the magnitude of the peak intensity of the absorption spectrum obtained from the detection signals from the detector 7 and the concentration of the sample component. However, the light received by the light-receiving elements is not always limited to the light emitted from the sample and dispersed into wavelengths by the concave diffraction grating 6; for example, the reflection of light from various optical components in the spectrophotometer falls onto the light-receiving elements as stray light. In such a case, the amount of light received by each light-receiving element becomes greater than that of the light within the intended wavelength range. In particular, when the concentration of the sample component is high, the light within a specific wavelength range undergoes considerable absorption, causing an increase in the proportion of the amount of stray light to that of the light intended to be received, so that the influence of the stray light becomes noticeable. Accordingly, various methods for removing such an influence of the stray light on spectrometric analyses have been proposed.
For example, Patent Literature 1 discloses a method for computing a component concentration in a sample cell in a spectrophotometer, using a cell divided into a sample cell and a reference sample cell by a separator window. In this method, the absorbance of light is detected for both the sample cell and the reference sample cell while the separator window is gradually moved. The component concentration is determined from the displacement of the separator window, absorbance of the sample cell and that of the reference sample cell. This removes the influence of the stray light originating from such components of light as reflected by components on the cell surface, scattered by scratches or stains on the cell, or scattered by dust or suspended matter floating in the sample.
Patent Literature 2 discloses a method for removing the influence of a stray-light component in a scanning range sensor in which an emission of light from a light source is cast into a sample through a mirror or half mirror and an emission of light from the sample is directed through a mirror or half mirror into a photodetector, the stray-light component being such light as reflected or scattered on the surface of a window section for admitting the light from the light source into the sensor. According to Patent Literature 2, the incidence of the stray-light component on the photodetector is prevented by adjusting the orientation and other parameters of the mirrors and half mirrors so as to direct the stray-light component onto a light absorber placed within the sensor.